In previous blogs we have discussed the role of protein intake for muscle protein synthesis. However, muscle contains many different functional proteins. While most research has focussed on contractile proteins, muscle also contains many connective proteins that play a very important role in transferring forces along the muscle. As much as 80% of the contractile force is transferred through the connective protein network before reaching the tendon to facilitate joint movement. These connective proteins are found both inside the muscle cells and well as on the outside of these cells.
Components of skeletal muscle
The muscle connective protein network is a key factor for the capacity of muscles to generate force. Just like other muscle proteins (contractile proteins or mitochondrial proteins) connective proteins are constantly being broken down and synthesised. Exercise (especially resistance exercise) will increase protein synthesis and it is thought that over time the connective protein network will become stronger in line with a muscle that is becoming stronger.
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Little research is available to study the effects of protein intake on connective tissue within the muscle. It has been suggested that perhaps the ingestion of protein and in particular collagen would help the synthesis of this connective tissue. An early study showed that proteins from food were indeed used for the synthesis of connective tissue proteins, protein intake immediately after resistance exercise had no additional effect (1). The protein that was ingested in this study and others have suggested that perhaps a different protein like collagen would be more effective.
Collagen and muscle connective tissue
Much of the muscle connective protein network consists of collagen strands, which are abundant in glycine (25%), proline (12%) and hydroxyproline (14%). It has therefore been hypothesised that connective protein synthesis rates after exercise is limited by the provision of ample glycine and proline precursors for collagen synthesis. In contrast to casein (or whey) protein, collagen provides ample amounts of glycine and proline that could support a greater increase in muscle connective protein synthesis after exercise.
A new study on collagen synthesis ratesÂ
A new study tested this hypothesis (2). 45 healthy young men and women were selected, and they ingested 30 g whey protein, 30 g collagen protein, or a noncaloric placebo after a single bout of resistance exercise. It was shown that a single bout of barbell squat exercise increases both the synthesis of contractile proteins as well as connective proteins.
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Whey protein ingestion had an additive effect and increased protein synthesis rates of contractile proteins even more. However, in contrast to the hypothesis, whey protein intake had no effect on connective proteins. The ingestion of collagen protein did not increase the synthesis of contractile proteins or connective tissue compared with the placebo.
The researchers did see a substantial increase in glycine, proline and hydroxyproline in the blood but this did not seem to affect protein synthesis of connective proteins. Some studies have found some promising and interesting results of collagen supplementation for tendon and bone, but muscle connective tissue seems unaffected by it. Maybe this is because these tissues (ligaments, tendons, and bone) have a much higher collagen content (85% collagen) when compared with muscle tissue (5% collagen).
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Of course, this is only one study, with one collagen supplement and one dose and it is possible that a higher dose or a different supplement would have worked better.
Summary
In conclusion, resistance exercise increases both synthesis of muscle contractile proteins as well as muscle connective proteins. Whey protein ingestion can help to increase protein synthesis of contractile proteins. Collagen protein ingestion has no effect on the synthesis of connective proteins.
Reference
Trommelen J Holwerda AM Senden JM, et al. Casein ingestion does not increase muscle connective tissue protein synthesis rates. Med Sci Sports Exerc. 2020;52(9):1983–91.
Aussieker T, Hilkens L, Holwerda AM. Collagen Protein Ingestion during Recovery from Exercise Does Not Increase Muscle Connective Protein Synthesis Rates. Med Sci Sports Exerc. 2023;55(10):1792-1802.
